An exhaust emission control apparatus typically includes a catalytic converter provided in an exhaust passage of an internal combustion engine to reduce the exhaust gas emissions produced by the engine. Such converters operate to chemically alter the exhaust gas composition produced by the engine to help meet various environmental regulations governing tailpipe emissions. Current methods of estimating the midbed temperature of a catalytic converter, however, fail to consider the effects of engine misfires which cause an increase in the temperature of the catalytic converter.
The use of an estimated temperature of the midbed point of a catalytic converter based on the rate of engine misfires is necessary for enhanced California Air Resources Board (CARB) On-Board Diagnostic (OBD-II) regulated systems. Under OBD-II automotive manufacturers must illuminate the malfunction indicator light (MIL) when the rate of misfires exceeds a rate which will cause catalyst damage due to excess temperature. Engines have misfires for many reasons soon after a cold start resulting in a high rate of illumination of the MILs. However, the catalyst has not been damaged because the engine has just been started. Thus, a large and unnecessary warranty cost is incurred.
Accordingly, there exists a need to accurately estimate the temperature of the midbed point of a catalytic converter taking into consideration the effect of engine misfires for use in determining when a rate of misfires exceeds a threshold which may cause catalyst degradation due to excess temperature.